1. Field of the Invention
The present invention relates to an apparatus and method for processing substrates. Specifically, the invention relates to a method for depositing a conformal layer of material on a substrate in an ionized metal plasma process.
2. Background of the Related Art
Sub-quarter micron multi-level metallization represents one of the key technologies for the next generation of ultra large-scale integration (ULSI) for integrated circuits (IC). In the fabrication of semiconductors and other electronic devices, directionality of particles being deposited is important in filling features. As circuit densities increase, the widths of vias, contacts and other features have decreased to 0.25 .mu.m or less, whereas the thicknesses of the dielectric layers remain substantially constant. Thus, the aspect ratios for the features, i.e., the ratio of the depth to the minimum lateral dimension, increases, thereby pushing the aspect ratios of the contacts and vias to 5:1 and above. As the dimensions of the features decrease, it becomes even more important to get directionality of the particles in order to achieve conformal coverage of the feature side walls and bottom.
One process capable of providing directionality to particles is ionized metal plasma-physical vapor deposition (IMP-PVD), also known as high density physical vapor deposition. A typical IMP-PVD chamber includes a coil for maintaining a high density, inductively-coupled plasma between a target and a susceptor on which a substrate is placed for processing. The plasma is generated by introducing a gas, such as helium or argon, into the chamber and then coupling energy into the chamber to ionize the gas. A magnetron assembly disposed behind the target provides magnetic field lines which trap electrons adjacent the target to cause increased ionization of the sputtering gas at the target surface. The constituents of the plasma are accelerated toward the target by a bias applied to the target causing the sputtering of material from the target by simple momentum transfer. Power supplied to the coil produces an electromagnetic field which induces currents in the plasma to intensify the density of the plasma and ionize the sputtered metal flux that traverses the electromagnetic fields generated by the coil. An electric field due to an applied or self-bias, develops in a boundary layer, or sheath, between the plasma and the substrate that accelerates the metal ions towards the substrate in a vector parallel to the electric field and perpendicular to the substrate surface. While the self-bias can be maintained by floating the susceptor relative to the plasma, the bias energy is preferably modulated by the application of power, such as RF or DC power, to the susceptor, thereby increasing the attraction of the sputtered target ions to the surface of the substrate in a highly directionalized manner to fill features in the substrate.
One of the problems encountered in IMP-PVD is an inability to achieve conformal coverage in the increasingly smaller device features. The directionality provided by the substrate bias results in good bottom coverage, but moderate side wall coverage. This result is caused in part by the induced directionality of ions toward the bottom of the features with little directionality toward the side walls. FIG. 1 illustrates the direction of ions 12 from a plasma 14 entering a via 16 formed on a substrate 10. The direction of the ions 12 is influenced by the electrical field E toward the bottom 18 of the via 16 resulting in little deposition on the side walls 20.
The limitations of IMP-PVD can result in non-conformal layers on device features. Conformal coverage of the bottom 18 and side walls 20 of the features is needed to optimize subsequent processes such as electroplating. For example, electroplating requires conformal barrier layers and conformal seed layers within the device features in order to ensure uniform filling of the feature. Thus, where the material is not conformably provided to the device features, subsequent processes, such as electroplating, may not be successful and result in the formation of defective devices.
Therefore, there is a need to provide a technique for providing better conformal coverage of device features, and in particular, better conformal coverage of side walls.